1. Field of the Invention
The invention relates generally to the field of seismic imaging of subsurface rock formations. More specifically, the invention relates to methods for determining hypocenters and focal mechanism of seismic events occurring in the Earth's subsurface.
2. Background Art
U.S. Patent Application Publication No. 2008/0068928 filed by Duncan et al. describes methods for determining the hypocenter (location of occurrence) of seismic events in the Earth's subsurface, and application of such methods to monitoring the progress of hydraulic fracturing operations. Such methods are generally known as “passive” seismic surveying because they do not use a separate, controllable seismic energy source to impart seismic energy into the subsurface, but use naturally occurring seismic events, or induced events such as breaking of rock formations by hydraulic fracturing. The described techniques in the Duncan et al. publication generally include deploying an array of seismic sensors on or near the surface, and recording signals produced by the sensors. The seismic signals are transformed into a domain of possible spatial positions of a source of seismic events. An origin in spatial position and time of at least one seismic event is determined from the space and time distribution of at least one attribute of the transformed seismic data.
An unavoidable aspect of passive seismic surveying is extended recording time in order to obtain sufficient signal to noise ratio. Such recording time may be several weeks to several months in length. In the case of hydraulic fracturing, the seismic events are limited in number and may not provide sufficient signal to noise ratio to be able to reliably identify hypocenters of the seismic events. It is desirable to have techniques for interpreting passive seismic signals that can improve signal to noise ratio.